Ultrafast quantum dynamics and resonance Raman spectroscopy of photoexcited I-2(B) in large argon and xenon clusters

The early quantum dynamics following the B((3) Pi(0u)+) <-- X photoexcitation of I-2 in large rare gas clusters is studied and the resonance Raman spectrum of these systems is calculated by a novel time-dependent quantum mechanical simulation approach. The method used is the classically based separable potential (CSP) approximation, in which classical molecular dynamics simulations are used in a first step to determine an effective time-dependent separable potential for each mode, then followed by quantum wavepacket calculations using these potentials. In the simulations for I-2(Ar)(n) and I-2(Xe)(n), with n = 17, 47, all the modes are treated quantum mechanically. The Raman overtone intensities are computed from the multidimensional time-dependent wavepacket for each system, and the results are compared with experimental data on I-2 in Ar matrices and in liquid Xe. The main findings include: (i) Due to wavepacket dephasing effects the Raman spectra are determined well before the iodine atoms hit the rare gas ''wall'' at about 80 fs after photoexcitation. (ii) No recurrencies are found in the correlation functions for I-2(Ar)(n). A very weak recurrence event is found for I-2(Xe)(n). (iii) The simulations for I-2(Ar)(17) (first solvation layer) and for I-2(Ar)(47) (second solvation shell) show differences corresponding to moderate cluster size effects on the Raman spectra. (iv) It is estimated that coupling to the B '' ((1) Pi(1u)) state or to the a (1 g) state have a small effect on the Raman intensities. (v) For I-2(Ar)(47), the results are in very good quantitive agreement with I-2/Ar matrix experiments. The I-2(Xe)(n) results are in qualitative agreement with experiments on I-2 in liquid Xe. The reported calculations represent a first modeling of resonance Raman spectra by quantum dynamical simulations that include all degrees of freedom in large systems, and they demonstrate the power of the CSP method in this respect. (C) 1996 American Institute of Physics.